Mono Ethylene Glycol (MEG) is used to prevent hydrate formation in pipelines transporting gas, condensate and water. It may also contribute to pipeline corrosion control. Typically, a 90 wt % MEG solution is injected to the gas stream at the beginning of the pipeline. A water phase may or may not be present at the pipeline inlet. As the stream cools down though the pipeline, water will condense from the gas. Water and MEG will mix completely and the mixture of aqueous methyl ethylene glycol is called rich MEG, because it is rich in water. The MEG concentration in the rich MEG can be 30-80 wt %, typically 50-65 wt %.
The rich MEG is separated from the gas and the condensate in one or several separation stages. Normally the rich MEG is heated to 30-80° C. to improve separation and avoid formation of emulsions/foam. Filters, centrifuges, decanters, coaleshers etc. may be included to improve the separation process and for removal or particles. The rich MEG is then normally sent to a storage/buffer tank, before it is sent to a regeneration unit to remove impurities. The processing of rich MEG is called pre treatment.
MEG is regenerated in a regeneration unit which typically consist of a reboiler/heater and a distillation column. The rich MEG is heated in the reboiler and most of the water is evaporated to produce the desired MEG concentration in the reboiler, normally around 90 wt %. The feeding point may be either directly into the reboiler or in the distillation column. The vapour is distilled to remove MEG and produce water with as low MEG concentration as possible, typically below 1000 ppm.
The rich MEG may contain many contaminants, such as ions, particles and various production chemicals. Especially troublesome are divalent cations such as iron, calcium, barium, strontium and magnesium because they can precipitate as various carbonates and hydroxide salts in the regeneration system. Carbonate salts from divalent cations tend to precipitate on hot surfaces because their solubility decreases with increasing temperature. To avoid scale problems in the reboiler, these ions should be removed from the rich MEG before entering the reboiler or other distillation equipment.
A solution for chemically removing certain salts from MEG is described in WO 2009/017971, which describes a reclamation unit that removes the salt as a pre-treatment before distillation, by adding chemicals such as NaOH, NaHCO3, Na2CO3 to increase pH and carbonate concentration. This will lead to high super saturation of the carbonate salts of the divalent cations and they will thus precipitate and can be filtered off.
If chemicals are added, they will accumulate in the MEG unless they are taken out later. Adding NaOH, NaHCO3, Na2CO3 or similar to increase pH means that sodium concentration increases and also the alkalinity. (It is also possible to use the similar potassium salts). A reclaimer stage may have to be included to control the salt concentration level in the loop. Having to use the reclaimer makes the process more complicated in addition to increased chemical costs and relatively high energy consumption.
Another solution is to precipitate polyvalent cations by preheating the MEG. This can be done by a preheater unit positioned before the distillation unit. Heating the rich MEG will lead to salt precipitation, which may subsequently be removed by some solids removal process such as filtering, centrifuge or settling. Heating alone may, however, be insufficient to get quantitative precipitation of the divalent cations and the reaction rate can be slow, requiring a large flash drum or similar tank to increase the retention time.